Conventionally, a focus detection method used in a camera includes a focus detection method based on triangulation used in a lens shutter camera, that based on contrast detection used in a video camera or the like, that based on pupil splitting used in a single-lens reflex camera, and the like.
In the focus detection method based on triangulation, infrared light is projected onto an object, and light reflected by the object is received by a sensor such as a PSD or the like via a light-receiving lens which is located at a position different from the projection light, thereby detecting a distance from that light-receiving position to the object (see Japanese Patent Publication No. 47-23929).
However, when the focus detection method based on triangulation is applied to a digital still camera, since a photographing optical system is different from a ranging optical system, a ranging region changes unwontedly when the photographing optical system undergoes zooming.
In the focus detection method based on contrast detection, a change in contrast of an image captured by an image sensor is detected while driving a photographing lens, so as to attain focus detection based on that change amount (see Japanese Patent Publication No. 39-5265).
However, since the focus detection method based on contrast detection detects a change in contrast of an image captured by the image sensor while moving the photographing lens, and detects a focus state based on that change amount, focus detection takes long time in, e.g., a state largely different from an in-focus state. Furthermore, this method cannot attain focus detection of a moving object.
In the focus detection method based on pupil splitting of a photographing lens, a focus state of the photographing lens is detected by calculating a correlation between two images generated by light beams which are transmitted through different pupil regions of the photographing lens (see Japanese Patent Laid-Open No. 63-18313).
Also, an optical apparatus which forms a part of a stop of a photographing optical system by a polarizing plate, and comprises a beam splitter, which guides some light components of a light beam outside an optical path so as to attain AF, between the stop and an image sensor, and a polarizing plate between the beam splitter and image sensor, has been proposed. This proposal discloses, as a second embodiment, an example in which the entire light beam is covered by a polarization beam splitter and the polarizing plate and polarization beam splitter are formed by identical components (see Japanese Patent Laid-Open No. 6-175010).
In this optical apparatus, even when stop blades are in a close state, a light beam which is transmitted through the portion formed by the polarizing plate reaches an AF sensor to attain AF. In addition, since the light beam is intercepted by the polarizing plate arranged between the beam splitter and image sensor, the close state is maintained on an image capturing plane. The same effect can also be obtained in the arrangement in which the polarization beam splitter covers the entire light beam.
FIG. 50 shows a schematic arrangement of the optical apparatus disclosed in Japanese Patent Laid-Open No. 6-175010 above.
Referring to FIG. 50, reference numeral 901 denotes a photographing optical system; 902, a light-receiving device such as a photo film, CCD sensor, or the like, and has sensitivity to only visible light; and 903, a beam splitter.
An image captured by the light-receiving device 902 is developed to form a photo original plate in case of a film camera, or is displayed on an electronic viewfinder (EVF), is recorded on a memory, or is printed out by a printer in case of a digital color camera.
A dielectric multilayered film is formed on a light splitting function surface 903a of the beam splitter 903, and the surface 903a reflects 50% visible light components of object light output from the photographing optical system 901 and transmits 50% remaining light components. Light reflected by the light splitting function surface 903a is totally reflected by a surface 903b of the beam splitter 903, and exits the beam splitter 903 via a surface 903c. 
An example that pertains to a digital single-lens reflex camera which forms a primary object image (objective image) formed by the photographing optical system on a two-dimensional light-receiving sensor such as a CCD sensor, CMOS process compatible sensor (to be simply referred to as a CMOS light-receiving sensor hereinafter), or the like, and photoelectrically converts the optical image to obtain an image output associated with an object is disclosed (see Japanese Patent Laid-Open No. 2003-140246).
This digital single-lens reflex camera as one of optical apparatuses includes a beam splitter which transmits light in a visible wavelength range without decreasing its light amount, and splits light in a wavelength range near an infrared range. Light in the wavelength range near the infrared range, whose optical path is split by the beam splitter, is used in focus detection, and light which goes straight undergoes image capturing.
By limiting the light splitting function surface of the beam splitter to a range through which a focus detection light beam passes, a low-profile beam splitter can be adopted, and can be laid out in a small space between the photographing optical system and a mirror which deflects an optical path to a viewfinder optical system without increasing the camera size. Since the spectral transmittance characteristics of the beam splitter are set to be nearly 100% in the visible wavelength range, a bright, high-quality image can be obtained without decreasing the light amount of the visible wavelength range required to capture an object image.
Since the optical apparatus (FIG. 50) disclosed in Japanese Patent Laid-Open No. 6-175010 adopts an arrangement for guiding only a light beam required for focus detection to the AF sensor, the beam splitter can be downsized. However, in practice, an image captured using straight traveling light suffers luminance nonuniformity unless the light splitting function surface of the beam splitter has a size as large as it can cover the entire light beam associated with image capturing, thus considerably deteriorating the image quality. When the beam splitter is arranged in the vicinity of a pupil plane of the photographing optical system, luminance nonuniformity hardly occurs. However, since the amount of light which is transmitted through the central portion of the pupil is reduced, the contrast of an image undesirably drops.
When this optical apparatus is, e.g., an infrared ray camera, the light-receiving device 902 in FIG. 50 has sensitivity to only infrared rays, and the light splitting function surface 903a of the beam splitter 903 is configured to split infrared rays. Since visible light transmitted through the light splitting function surface 903a is attenuated to ½, as described above, even if an object to be captured is a uniform luminance surface, an image captured by the light-receiving device 902 becomes like an image 910 on which upper and lower bright regions 912 and 913 are formed to sandwich a central dark region 911, as shown in FIG. 51.
Such phenomenon undesirably results in an image on which an unnatural luminance difference is conspicuous in a portion which should have uniform luminance and whose image quality deteriorates considerably, when blue sky, a white wall of a building, or the like is captured.
To solve such problems, an optical apparatus in which extinction parts 905a and 905b are arranged in front of the beam splitter 903, as shown in FIG. 52, has been developed. In this optical apparatus, when gaps are formed between the extinction parts, luminance nonuniformity as shown in FIG. 53A occurs; when overlapping portions are formed between the extinction parts, luminance nonuniformity as shown in FIG. 53B occurs.
In the second embodiment of the optical apparatus disclosed in Japanese Patent Laid-Open No. 6-175010, the polarization beam splitter becomes bulky since it must cover the entire light beam. As a result, large dimensions are required between an optical lens group and image sensor, and it is difficult to attain a size reduction of the apparatus.
The example disclosed in Japanese Patent Laid-Open No. 2003-140246 will be described below. In general, in the optical structure which splits an optical path of a light beam fetched via the photographing optical system into a plurality of light beam components, and guides them to the light-receiving device, advantages are often provided when the plurality of split light beam components have substantially the same wavelength characteristics.
In the camera disclosed in Japanese Patent Laid-Open No. 2003-140246, since light in the wavelength range near the infrared range, which is split by the beam splitter, is used in focus detection, aberration correction of the photographing optical system must be done in this wavelength range so as to correctly function the focus detection.
If the aberration correction is insufficient, it is impossible to strictly adjust a focus in the visible wavelength range using the light near the infrared range. On the other hand, when aberration correction is to be made for a range near the infrared range in addition to the visible wavelength range, a measure such as use of special glass, an increase in the number of lenses which form the photographing optical system, or the like is undesirably required, resulting in increases in cost and size. Especially, when the photographing optical system is exchangeable like in the single-lens reflex camera which comprises large-scale exchangeable lens systems, all the exchangeable lens systems must support such focus detection system, and it is very difficult to realize such system.
When the exposure value upon image capturing is to be determined by measuring the object luminance using a light beam split by the beam splitter, a phenomenon similar to that of focus detection occurs. More specifically, when the wavelength range for luminance measurement shifts from that for image capturing, since it is difficult to strictly estimate light energy included in the wavelength range for exposure from that included in the wavelength for luminance measurement, if image capturing is done by determining the exposure value on the basis of the measurement result of the object luminance, an underexposure or overexposure image capturing result may occur.